1. Field of the Invention
The present invention relates to the field of multi-shaft gas turbine engines, for aviation application in particular, and is aimed at a means of offtaking mechanical power from the rotor shafts in order to drive accessories.
2. Description of Related Art
A gas turbine engine usually comprises an air compressor set at least partially supplying a combustion chamber. The gases from the latter drive one or more turbines mechanically connected to the compressors, and supply the thrust. A double-shaft engine comprises a “low pressure” compressor set connected via a first shaft to a group of low pressure turbines, the assembly forming the low pressure, LP, shaft. It also comprises a second shaft, called the high pressure, HP, shaft, fixedly attached to a second shaft concentric with the first. The two rotors are mechanically free to rotate relative to one another. The HP shaft is in direct communication with the combustion chamber. Turbojets, civil aviation turbojets in particular, usually comprise a fan rotor driven by the LP shaft and delivering a large part of the thrust.
A part of the power supplied by aviation turbine engines is used to supply the auxiliary machines both of the turbine engines themselves and of the aircraft of which they provide the propulsion.
In a multi-shaft engine, this power is currently taken off partly mechanically from the shaft of the high pressure stage to drive a receiving input shaft of a gearbox for driving the accessories. This gearbox is also called the AGB (for accessory gear box). For a turbofan turbojet for example, the AGB gearbox is placed on the fan casing. Its input shaft is usually driven by a transmission shaft housed in one of the structural arms of the casing and connected through an angle transmission gearbox to a pinion fixedly attached to the high pressure shaft. Various accessory machines, such as generators and hydraulic fuel or oil pumps, are installed in this gearbox and driven by means of gear sets.
Another part of the offtake consists of the pressurized air bled off from the high pressure compressor to provide in particular the pressurization and air conditioning of the aircraft cabin or else the de-icing.
The current trend is aimed at increasing the proportion of the mechanical power offtake due to the increasing proportion of the electrical means, considered more flexible to use. This increasing demand for electrical supply for the equipment of the aircraft no longer makes it possible, for reasons of engine operation and performance, mainly at low rotation speeds, to take off the power from the HP shaft only. Such an offtake would be likely to cause surging of the compressor.